Rectification of gases



April 2, 1963 F. JAKOB 3,083,544

RECTIFICATION 0F GASES I Filed Sept. 25, 1959 3 Sheets-Sheet 1 Fig. I

' April 2, 1963 F. JAKOB 3,083,544

RECTIFICATIQN OF GASES Filed Sept. 23, 1959 S'ShQetS-Sheet 2 Fig.2

April 2, 1963 F. JAKOB 3,083,544

RECTIFICATION OF GASES Filed Sept. 23, 1959 3 Sheets-Sheet 3 ilnited rates Patent:

3,683,544 Fatented Apr. 2, 1$3

3,083,544 RECTIFICATIGN 0F GASES Fritz Jakob, Puliach, near Munich, Germany, assignor to Gesellschatt fur Lindes Eismaschinen Alrtiengeselischatt Hoilriegelkreuth, near Munich, Germany, a company of Germany Filed Sept. 23, 1959, Ser. No. 841,749 Claims priority, application Germany Sept. 24, E58 10 Claims. ((11. 62-28) This invention relates to the art of rectifying gaseous mixtures, and is concerned with the provision of a process and apparatus for obtaining a balanced cold economy in the production of gas mixtures and/ or gas mixture components, which are under a higher pressure, through rectification.

The known methods for obtaining a balanced cold economy in the production of oxygen, which is under an elevated pressure, involve the rectification of air by extraction in the liquid state and bringing it to the desired gaseous pressure state, for example, tempered to ambient air; compressing an amount of gastal en from the preliminary rectification and heated again in counterfiow in the heat exchangers for the incoming air-to a high pressure, before it is fed to the liquid oxygen, compressed to a higher pressure, for its evaporation and heating; and transforming it subsequently (if necessary, after additional heat exchange and expansion) into the preliminary or lowpressure rectification. This method requires the arrangement of high-pressure compressors whose particular disadvantage is the contamination of the gases by oil and moisture. Besides, the high-pressure compressors are very expensive, and are uneconomical in operation.

According to the present invention, the method for obtaining a balanced cold economy in the production of compressed gaseous mixtures and/ or gas mixture compon cuts from the rectification of one or several gas mixtures by extraction in the liquid low-pressure stage and by bringing them to the desired gaseous, mediumor high-pressure state, for example, tempered to ambient air, is characterized in that an amount of gas is taken from the preliminary rectification, heated in counterfiow to the incoming gas mixture, cooled by heat exchange for evaporation and heating of the liquid component, compressed to a higher pressure, and subjected to partial heating and subsequent work-producing expansion, after the liquefied components have been separated and returned. In contrast to the existing methods, this new method has the advantage that the gas taken from the preliminary rectification is brought, without pressure increase, by heat exchange to the component compressed to a higher pressure, and expanded work-producingly after partial heating. This gas is thus maintained in its pure state and requires no further treatment. It is not particularly characteristic of the process according to the invention where the expanded gas is conducted, but preferably it can be conducted to the lowpressure rectification or at least partly exhausted, if nec essary after further heat exchange in counterfiow with production gases, and after heating in the regenerators, etc. The partial heating of the gas before the workproducing expansion is effected preferably by heat exchange with a compressed gas of a so-called closed or open medium-pressure circuit. The quantity of gas effecting the partial heating for the work-producing expansion has its pressure reduced and is subjected to rectification, for example, in the preliminary or in the low-pressure stage. Compression under medium pressure preceded by heating and followed by cooling of the gas in counter-flow to itself conveniently forms a closed medium-pressure circuit. But if the gas which efiects the partial heating of the gas destined for work-producing expansion is compressed to a pressure suitable for obtaining the desired compressed gas mixture under higher pressure, liquefied, supercooled, fed to the rectifying and washing device for said gas mixture and then mixed with the resulting gas mixture, one has an example of an open medium-pressure circuit. These medium-pressure circuits, in which the pressure rarely attains or exceeds 30 atmospheres absolute, are of particular advantage because of the possibility of using dry-running compressors, since the gas to be compressed is not contaminated and, therefore, needs no after-treatment or complicated purification.

An open medium-pressure circuit may, however, be accompanied by a displacement of cold, so that in order to return it from the rectification of one gas mixture to that of the other, an amount of gas must be extracted from the preliminary rectification, liquefied in indirect heat-exchange against the gas mixture formed by the supply of a part of the liquefield gas effecting the partial heating, and subjected again to a preliminary rectification. Another possibility of exchanging cold between the two gas mixtures taken from a rectification of each consists in that the complete heating of the component brought to a higher pressure and the preliminary cooling of the compressed gas mixture which is to be separated is eifected in mutual heat exchange, preferably by the interposition of a so-called brine circuit, because of the fire and explosion hazard.

Embodiments of the invention are represented in the accompanying drawings to enhance the understanding. Similar parts have been designated with the same references.

In the drawing,

FIG. 1 is a diagrammatic representation of an apparatus organization for carrying out the above-described process, with particular applicability to the separation of the components of air;

PEG. 2 is a diagrammatic representation of a modified form of the apparatus organization shown in FIG. 1; and

FIG. 3 is a diagrammatic representation of a further modification of the apparatus organization shown in FIG. 1, with particular applicability to the provision of compressed nitrogen for production of synthetic ammonia gas.

FIGURE 1 shows the most important parts of an apparatus for the decomposition of gas mixtures, air being used as the example.

The apparatus consists essentially of an intake pipe 1 for air compressed to about 5.3 to 6.5 atmospheres absolute; a pair or" periodicallly reversible regenerators 2 for cooling the incoming air; a connecting pipe 3 to the sump of a rectifying column 4, with a pressure column 5; a lowpressure column 6 and a condenser 7; a line 8, with an expansion valve 9, from the sump of the pressure column 5, to the low-pressure column 6 for the liquefied gas mixture; a pipe ill, with an expansion valve 11, from the pressure side of the condenser '7 to the head of the lowpressure column 6, for liquefied nitrogen with connected counter-flow heat exchanger 12, an extraction pipe extending through this to the regenerators 2; a second interposed counterfiow heat exchanger 14; a discharge pipe 15 for the nitrogen produced; an extraction pipe 16, from the low-pressure part of the condenser 7, for liquid oxygen; a liquid gas pump 17 for compressing oxygen to the desired higher pressure; a pressure pipe 18 to a counterfiow heat exchanger 19, in which the liquefied component (here oxygen), compressed to a higher pressure, is heated; and an oxygen pipe 29 with an addition to the apparatus according to the invention, which is characterized by an extraction pipe 21 from the pressure column 5, to the regenerators 2 for extracting and heating air from the pressure column, heating coils 22 in the regenerators 2; a line 23 to the counterfiow heat exchanger 19; a postconnected liquid separator 24, with a return pipe for the condensate and a gas pipe 26, to a heat exchanger 27, Whose opposite faces are under medium pressure; a feed pipe 28; an expansion turbine 29; an outlet 30 with a branch pipe 31; an extraction pipe 32 for air from the pressure column; a counterflow heat exchanger 33 for heating this air; a pipe 34; a dry-running compressor 35 with a post-connected cooling device; a pipe 36 to the pounterflow heat exchanger 33 for cooling the air compressed to medium pressure; a pipe 37 to the heat exchanger 27 for partial heating of the work-producing, expanding air; a discharge pipe 38 with an expansion valve 39, to the pipe 3 and into the pressure column 5, respectively; and a pipe 49 with a cut-oil member 41 is provided between the pipes 21 and 26 to transfer compressed air, if necessary.

The medium-pressure circuit, represented by the parts .32, 33, 34, 35, 3'6, 37, 38 and 39, which is self-contained by the extension into the parts 3, 5 and 21, is arranged because the regenerators 2 work with excess discharge .so that there is no excess heat available that can be used for heating the air to be expanded in the turbine 29. The partial heating of the air to be expanded in the turbine 29 is therefore effected in the counter-flow heat exchanger 27, which is interposed in the medium pressure circuit, by taking over the heat of liquefaction from the air conducted in the medium-pressure circuit. If the amount of air issuing from the separator 24 is not sufli c-ient to produce the necessary cold in the work-producing expansion taking place after the partial heating, the amount is increased by adding pressure-column air by way of the pipe and the valve 41. Through'the pipe 3% a portion (usually a small portion) of the expanding air is injected into the low-pressure column, another (usually larger) portion is added to the nitrogen issuing through the pipe 13 by Way of the branch line 31.

FIGURE 2 shows, in a practically identical device for carrying out the same process, a'variation in the arrangement of the medium-pressure circuit, according'to which variation instead of a medium-pressure circuit for air from the pressure column, a closed medium-pressure circuit for nitrogen from the pressure column-is provided. .Nitrogen is extracted from the condenser 7, through the pipe 51, of the pressure column 5: it is heated in the heatexchangerSZ and conducted through the line 53 to 'the medium-pressure, dry-running compressor 54, aftercooled and cooled by the line 55 in the heat exchanger 52 in counterflow to itself, conducted through the line 56 to the counterflow heat exchanger 27 for liquefaction, where the partial heating of the air to be expanded is eflected, and returned to the pressure column 5 through the line 57 and the expansion valve 58.

The embodiment according to FIGURE 3 shows the connection of an air separation plant 61 for the production of compressed oxygen and compressed nitrogen with a gas separation plant 62, for the production of synthetic ammonia gas from converter gas which consists of hydrogen, nitrogen, carbon monoxide and methane. The oxygen is to be supplied with a pressure of about 40 atmospheres, and the nitrogen is to be brought to the pressure of the crude gas, that is, about 25 atmospheres. For heating the turbine gas there is used the nitrogen which must, in any event, be transferred to the gas plant, that is, the production nitrogen. The production nitrogen is extracted in the line 63 from the pressure column 5, under its pressure, conducted in counterfiow to the incoming air in spirals 64 and heated, brought outside the separation apparatus, for example, by means of the dry-running compressor 65 to about 25 atmospheres absolute, andis subjected to a heat exchange, after recooling in an after-cooler (not shown) in counterflow with the residual gas arriving from the gas plant and with the hydrogen-nitrogen mixture. It is cooled, in the counterflow heat exchangers: and 67, approximately to the start of its liquefaction, and then liquefied in the counterflow heat exchanger 68, thereby giving ofi" again (as in the preceding example) its heat of liquefaction to the air to be expanded in the turbine 29. This nitrogen, which is liquefied in the counterfiow heat exchanger 68, is slightly supercooled in the counterflow heat exchanger 69 against residual gas, and then arrives through the line 70 in the liquid state in the gas plant 62, where one part of is used for washing in the wash column 71 while the other part is fed directly to the hydrogen-nitrogen mixture in 72.

A great amount of liquid nitrogen has thus been fed to the gas apparatus 62, much more than it needs to compensate for its own cold losses. A corresponding amount of liquid must, therefore, be again removed from the apparatus 62 in order to balance its cold economy. To this end, nitrogen is extracted from the pressure column 5 and is fed to the counterfio-w heat exchanger 72.. In counterflow heat exchanger 72 this nitrogen, taken from the pressure column 5, is liquefied by heat exchange with the liquid nitrogen evaporating in the hydrogen mixture, and is introduced again into the air plant. The cold, which the liquid nitrogen has conducted from the counterflow heat exchanger 69 to the gas apparatus 62, has now returned to a great extent to the air separation plant 61, and only so much is left in the gas plant 62 as is necessary for compensating for the cold losses in the gas plant 62.

The crude gas is cooled in the counterflow heat exchangers 73 and 74, in counterflow with the hydrogennitrogen mixture issuing through the line 75. a The crude gas arrives in line 76, and is cooled in a counterflowprecooler 73 to about '5? C. and the condensed water is stripped off in the separator 77. In' the following counterflow-cooler 74, the gas is cooled to about 5 0 C. It then enters a gel drier and from there-in the counterflow heat exchangers proper 79 and 8.0, namely the hot leg (79) and the cold leg (80). From the cold leg 80 it is V transferred to the evaporating vessel 81, where it is cooled to a constant temperature of about K. by means of liquid nitrogen or residual gas evaporating .without pressure. The crude gas arrives at this temperature through the line 82 in the wash column 71, and is here converted to pure hydrogen-nitrogen mixture. The washed-out'co'nstituents leave the wash column 71 at the sump 83' as residual gas in liquid form through the line' 84. This liquefied residual gas is fed to the evaporating vessel 81 and is used there for cooling the crude gas, another portion arrives in liquid form in the air separation plant 61 and is heated there in the counterflow heat exchanger 69 and in the counterfiow heat exchanger 66, against the entering medium-pressure nitrogen. The heated residual gas leaves the plant 61 byway of the line 85. -In order to utilize the cold still contained in the oxygen after passing through the heat exchanger 19, a brine circuit is interposed between the oxygen line 29 and the counterflow heat exchanger 73 and 74 of the gas separation part. This brine circuit 86 transfers the cold-still contained in the oxygenover the heat exchanger 37 to the crude gas and serves to precool the crude gas. Because the compressed oxygen leaves the counterfiow heat-exchanger "19 relatively cold in this method of low-pressure heat e x- 'change, the great cold supply can be utilized with advantage and the otherwise necessary cooling by ammonia, which requires energy, can be avoided.

I claim:

1. In a process of separating compressed gas mixtures by rectification in a preliminary rectification column and at least one further rectification column at low temperature and the production of high pressure gases thereof,

the method of producing cold which comprises withdrawing a component of a gas mixture to ,be separated from a rectifying column at low' pressure in the .liquid's'tate,

pumping said liquid to said high pressure;-w ithdrawing a gas from the preliminary rectification column and heating it in counter-flow to the incoming gas mixture, vaporizing said high pressure liquid .byheat-exchange with said gas,

thereby condensing said gas at least partially to form a liquid part and a gaseous part thereof, separating the gaseous part from the condensed and liquefied part of said gas, leading the liquefied part of said gas into said preliminary rectification column, warming the gaseous part of said gas and expanding it by the production of external work.

2. In a process of the separation of compressed air by rectification in a preliminary rectification column and at least one further rectification column at low temperature and the production of high pressure oxygen thereof, the method of producing cold which comprises withdrawing oxygen from a rectifying column at low pressure in the liquid state, pumping said liquid to said high pressure; withdrawing a gas from the preliminary rectification column and heating it in counter-flow to the incoming gas mixture, vaporizing said high pressure liquid by heatexchange with said gas, thereby condensing said gas at least partially to form a liquid part and a gaseous part thereof, separating the gaseous part from the condensed and liquefied part of said gas, leading the liquefied part of said gas into said preliminary rectification column, warming the gaseous part of said gas and expanding it by the production of external work.

3. Apparatus for the separation of gas mixtures comprising a conventional gas separation device with a twocolumn rectifying device and a regenerator arrangement for heat-exchange and cooling of the gas mixture, at gas conduit connecting the lower part of the high pressure column of said rectifying device with the cold ends of heating coils inside the regenerators, a pump connected to the sump of the low pressure column, a heat-exchanger for evaporating the high pressure liquid, means connecting the high pressure side of said pump to said heat-exchanger, means connecting the warm ends of said heating coils to the second channel of said heat-exchanger, means connecting the cold end of said second channel to a liquid separator, conduit means for returning the condensate from said separator to the pressure column, gas lines from the upper end of said separator to the cold end of a second heat-exchanger, means for passing compressed gas through the second channel of said second heat-exchanger, means connecting the warm end of said second heat-exchanger with the inlet of an expansion turnine and means connecting the outlet of said turbine with the low pressure column.

4. Apparatus as claimed in claim 3 in which said second channel of said second heat-exchanger is connected into a closed pressure circuit, consisting of a compressor and at least one further heat-exchanger.

5. Apparatus as claimed in claim 3 in which said second channel of said second heat-exchanger is connected into a pressure circuit, the pressure circuit consisting of the high pressure column of the two-stage rectifying device, means for conducting gas from said high pressure column through a first channel of a heat-exchanger to a compressor combined with a cooler, means for conducting high pressure gas from the compressor combined with a cooler through a second channel of said heat exchanger and from said following second channel of said second heat-exchanger to an expansion device, means for conducting expanded gas from said expansion device to said high pressure column.

6. Apparatus according to claim 3 in connection with another gas separation unit consisting of heatexchangers for cooling a gas to be washed and a washing device, the connection consisting of means connecting the head of the high pressure column of the conventional gas separation device with heating coils contained in the regenerators, means connecting the warm ends of said coils with a compressor and an aftercooler, means connecting the aftercooler with a heat-exchanging line conducting through one first heat-exchanger as well as another first heat-exchanger connected to the one heat-exchanger in parallel and from there through said second heat-exchanger and from there through a third heat-exchanger, means connecting said heat-exchanging line with an expansion device and with a condenser of said washing device, means connecting said expansion device with said high pressure column, means connecting the foot of the washing tower of said other separation unit with said third heat-exchanger, means connecting said third heatexchanger with the one of said two first-exchangers, means connecting said washing device with the other of said two first heat-exchangers.

7. "In a process of separating a compressed gas mixture by rectification in a preliminary rectification column and at least one further rectification column at low temperature and the production of high pressure gases thereof, cooperating with a process of rectification and purification of a compressed crude synthesis gas mixture to be washed with a liquid component of said gas mixture to be separated, which comprises withdrawing said gas from a rectifying column at low pressure in the liquid state; pumping said liquid to said high pressure; withdrawing a gas from the preliminary rectification column and heating it in counter-flow to the incoming gas mixture; vaporizing said high pressure liquid by heat-exchange with said gas, thereby condensing the latter at least partially to form a liquid part and a gaseous part thereof; separating the liquefied part and leading it into said preliminary rectification column; warming the gaseous part of said gas and expanding it by the production of external Work; withdrawing a higher boiling component from the rectification column and heating it in counter-flow to said incoming compressed gas mixture; compressing said component and cooling at least part of said compressed gas by heat-exchange with the residual gas withdrawn from a further rectifying and washing device employed for washing a compressed crude gas mixture; liquefying said compressed gas by heat-exchange with the gas to be expanded, warming said gas thereby; supercooling said liquefied compressed gas by heat-exchange with said residual gas and passing it into said rectifying and washing device and washing therein said compressed crude gas mixture, an amount of cold being returned from the rectifying and washing device for said compressed crude gas mixture to the rectification column of line other gas mixture by withdrawing a quantity of gas from the preliminary rectification column of the latter, liquefying said gas by indirect heat-exchange with the washed compressed gas mixture, and passing said liquefied gas into the preliminary rectifying column.

8. Apparatus for the separation of gas mixtures comprising a conventional gas sepanation device with a twocolumn rectifying device and a regenerator arrangement for heatexchange and cooling of the gas mixture in connection with another gas separation unit consisting of heat exchangers for cooling a gas to be washed and a washing device, a gas conduit connecting the lower part of the high-pressure column of said rectifying device with the cold ends of heating coils inside the regenerators, a pump connected to the sump of the low-pressure column of said rectifying device, a heat-exchanger for evaporating high-pressure liquid, means connecting the highpressure side of said pump to said heat-exchanger, means connecting the warm ends of said heating coils to a second channel of said heat-exchangers, means connecting the cold end of said second channel to a liquid separator, conduit means for returning condensate from said separator to the pressure column, gas lines from the upper end of said separator to the cold end of a second heatexchanger, means for passing compressed gas through a second channel of said second heat-exchanger, means con meeting the warm end of said second heat-exchanger with the inlet of an expansion turbine and means connecting the outlet of said turbine with the low-pressure column, means connecting the head of a high-pressure column of said conventional gas separation device with heating coils contained in the regenerators, means connecting the warm ends of said coils with a compressor and an aftercooler, means connecting the aitercooler with a heat-exchanging line conducting through one first heat-exchanger tasiwell as another first heat-exchanger connected to the one heatexchanger in parallel and from there through said second heat-exchanger and'from there through a third heatexchanger, means connecting said heat-exchanging line with an expansion device and with a condenser of said washing device, means connecting said expansion device with said high-pressure column, means connecting the foot of the washing towerof said other separation unit with said third heat-exchanger, means connecting said third heat-exchanger with the one of said two first heatexchangers, means connecting said washing device with the other of said two first heat-exchangers, means connecting a line for withdrawing part of a separation prodnot from 'the top of the high-pressure column, a first heat-exchange channel in an evaporation heat-exchanger of said other gas separation unit and a line for the return of said separation product liquefiedthe-rein to the top of said high-pressure column, the second channel of said evaporation-heat-exchanger being connected with the top of said washing tower and with said heat-exchanging line back of said third heat-exchanger and with said other heat-exchanger of said two first heatexchangers, and with the heat-exchanger cooling the :gas to 'be Washed, said apparatus further including a heat-exchanger one chanml of which is connected in series with heat-exchangers of a' precooling arrangement for the compressed crude 'g'as mixture'to be washed and the other channel of which is connected to the heat-exchanger in which the component pumped to higher pressure is evaporated.

9. In 'a process of separating a compressed gas mixture by rectification in a preliminary rectification column and at least one further rectification column at low temperature and the production of high pressure gases thereof, cooperating with a process of rectification and [purification of a compressed crude synthesis gas mixture to be washed with a liquid component of'said ga s mixture to be separated, which comprisesvvithdnawing a lower boiling component of the gas mixture to he separated from a rectifying column at low pressure'in liquid state; pumping the liquid to said high pressure; withdrawing a gas from the preliminary rectification column and heating it in counter-flow to the incoming gas mixture; vaporizing said high pressure liquid by heat exchange with said gas, thereby condensing said gas at least partially to form a liquid part and :a gaseous part thereof; separating the gaseous part from the condensed and liquefied part of said gas; leading the liquefied part of said gas into said preliminary rectification column;

warming the gaseouspart of said gas and'expandingi t by the production of external work; withdrawing a higher boiling component, fromthe rectification columnand heating it in counter-flow to said incoming compressed gas mixture; compressing said component and cooling it at'least partly by heat-exchange with a residual gas, the residual gas withdrawn from a device of said purification of said crude gas; liquefying said higher boiling component withdrawn from said rectification column by heatexchange' with the gas to 'be expanded; supercooling said liquefied higher boiling component by heat-exchange with said residual gas; passing at leastpart of said supercooled liquid into said device of said purification; and washing therein said compressed crude gas mixture.

10. Apparatus for the separation of gas mixtures comprising .a conventional gas separation device with a twocolumn rectifying device and a regenerator arrangement for heat-exchange and cooling of the gas mixture in connection with another gas separation unit consisting of heat exchangers for cooling a gas to be 'washed and a washing device, a gas conduit connecting the lower part of the high-pressure column of said rectifying device with'the cold ends of heating coils inside the regenenators, a pump connected to the sump of the low-pressure column of said rectifying device, a heat-exchanger for evaporating high-pressure liquid, meansconnecting the high-pressure side of said pump to said heat-exchanger, means connecting the warm ends of said heating coils to a second channel of said heat-exchangers, means connecting the cold end of said second channel 'to a liquid separator, conduit means for returning condensate from said separator'to the pressure column, gas lines from the upper end of said separator to the cold end of a second heat-exchanger, means for passing compressed gas through a second channel of said second heat-exchanger, means connecting the warm end of said second heatexchanger with the inlet of an expansion turbine and means connecting the outlet of said turbine with the lowpressure column, means connecting the head of a high pressure column of said conventional gas separation device with heating coils contained in the regenerators, means connecting the warm ends of said coils with a compressor and an 'aftercooler, means connecting the aftercooler with a heat-exchanging line conducting through one first heat-exchanger as well as another first heat-exchanger connected to the one heat-exchanger in parallel and from there through said second heat-. exchanger and from there through a third heat-exchanger, means connecting said heat-exchanging line with an expansion device and with a condenser of said washing device, means connecting said expansion device with said high-pressure column, means connecting the foot of the washing tower of said other separation unit with said third heat-exchanger, means connecting said third heatwashing tower and with said heat-exchanging line back of said third heat-exchanger and with said other heat-' exchanger of said first two heat-exchangers, and with the hea t -exchanger cooling the gas to be washed.

References Cited in the file of this patent UNITED STATES PATENTS 2,122,238 Poll-itzer June 28, 1938 2,708,831 Wilkinson May 24, 1955 2,785,548 Becker Mar. '19, 1957 2,822,675 Grenier Feb. 111, i958 2,827,775 Linde Mai. 25, 1958 2,895,304 Wucherer et al; July 21, 1959 FOREIGN PATENTS 884,203 Germany June 11, 1953 

1. IN A PROCESS OF SEPARATING COMPRESSED GAS MIXTURES BY RECTIFICATION IN A PRELIMINARY RECTIFICATION COLUMN AND AT LEAST ONE FURTHER RECTIFICATION COLUMN AT LOW TEMPERATURE AND THE PRODUCTION OF HIGH PRESSURE GASES THEREOF, THE METHOD OF PRODUCING COLD WHICH COMPRISES WITHDRAWING A COMPONENT OF A GAS MIXTURE TO BE SEPARATED FROM A RECTIFYING COLUMN AT LOW PRESSURE; WITHDRAWING A PUMPING SAID LIQUID TO SAID HIGH PESSURE; WITHDRAWING A GAS FROM THE PRELIMINARY RECTIFICATION COLUMN AND HEATING IT IN COUNTER-FLOW TO THE INCOMING GAS MIXTURE, VAPORIZING SAID HIGH PRESSURE LIQUID BY HEAT-EXCHANGE WITH SAID GAS, THEREBY CONDENSING SAID GAS AT LEAST PARTIALLY TO FORM A LIQUID PART AND A GASEOUS PART THEREOF, SEPARATING THE GASEOUS PART FROM THE CONDENSED AND LIQUEFIED PART OF SAID GAS, LEADING THE LIQUEFIED PART OF SAID GAS INTO SAID PRELIMINARY RECTIFICATION COLUMN, WARMING THE GASEOUS PART OF SAID GAS AND EXPANDING IT BY THE PRODUCTION OF EXTERNAL WORK. 